Limitations to Fruit Production in Ionopsis utricularioides (Orchidaceae)

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Abstract
The proportionally low fruit set observed in many epiphytic tropical orchids usually has been attributed to insufficient pollination; for example, the tropical orchid Ionopsis utricularioides, a twig epiphyte, offers no floral rewards, is rarely visited, and has low fruit set. To determine what factors limit fruiting in this species, we assessed effects of resources, pollinator service, pollen parentage, and pollen quantity using greenhouse and field studies in which individual plants were followed over two flowering seasons. Plants were self-compatible, and pollination frequency substantially affected fruit set. Hand-pollination of field experimentals increased total inflorescence fruit sets from 6 to 19 percent, even though over one-third of such pollinations failed to yield fruit. For flowers pollinated on the same day, fruit failure was related to leaf length but not to position in the inflorescence, flower age, or pollen quantity (one vs two pollinia). High fruit set resulted in reduced growth and a higher probability of foregoing inflorescence production the following year. This last result suggests that resource availability may limit lifetime fruit production, even though pollination limitation occurs within a single season. We also explored some predicted consequences of resource-limited reproduction. First, we found no evidence for mate choice in the sense of selective abortion of fruits depending on selfed verses outcrossed pollen parentage. Second, our plants produced many more flowers than could mature fruit. Although fruit production on a per-flower basis decreased with inflorescence size, pollinarium removal increased with number of open flowers. This suggests that pollen donation by surplus flowers increases the fitness of those plants with larger inflorescences. However, most inflorescences are small, which suggests trade-offs between conflicting selective pressures. Third, we speculate that resource limitation of fruit production may have influenced the evolution of nectar-deception pollination in many orchids.